1. Field of the Invention
The present invention relates to an agitating apparatus for an anaerobic digestion tank of sludge.
2. Description of the Related Arts
FIG. 5 is a longitudinal sectional view showing an entire construction of one example of a digestion tank to be used for an anaerobic sludge treatment of the prior art, as disclosed in Japanese patent examined application No. 46654/84 and FIG. 6 is a longitudinal sectional view showing the details of the mechanical agitating apparatus in an enlarged scale.
This digestion tank 1 is equipped therein with a draft tube 2 which is erected upright so that it can be submerged in the inside sludge. A screw 4 of an agitator 3 is inserted into the draft tube 2 from the upper open end so that downward (as indicated by solid arrows in FIG. 5) or upward (as indicated by broken arrows in FIG. 6) flows can be generated in tile draft tube 2 by rotating tile screw 4 which generates circulation flows in the tank by forced convection to agitate the sludge 5 as a whole. In other words, the draft tube 2 and the screw 4 function like a vertical axial flow pump using the screw 4 as its impeller and the draft tube 2 as its casing.
More specifically, the draft tube 2 has its lower portion fixed in the inner wall of the lower portion of the digestion tank 1 through a plurality of legs 6, as shown in FIG. 5, and its upper portion fixed in the inner wall of the upper portion of the digestion tank 1 through tie rods 7. As a result, the draft tube 2 can suck the sludge from its one open end and can discharge the sucked sludge from its other open end.
The agitator 3 is constructed such that a motor 11 is arranged outside of a bearing box 10 which is buried by mortar 9 in a concrete slab 8 of the digestion tank 1 just above the draft tube 2, as shown in FIG. 6, and such that the output shaft of the motor 11 and the shaft 12 of the screw 4 are connected through the bearing box 10.
Moreover, the shaft 12 is equipped near the sludge liquid level with a splash disc 13. Use of the splash disc 13 presents the sludge from being discharged from the upper open end of the draft tube 2 above the sludge liquid level, when the upward flow is established in the draft tube 2, and this prevents contamination of the bearing box 10 with the discharged sludge.
Here, an upper space 14 in the digestion tank 1 is filled up with the combustible digestion gas (under a pressure of about 300 mm Aq) which is composed mainly of methane gas released during the digesting step.
FIG. 7 is a longitudinal sectional view of the entire apparatus of another example of a digestion tank of the prior art to be used for sludge treatment in the prior art. The portions corresponding to tile aforementioned ones of FIGS. 5 and 6 are designated at the common reference numerals.
In this digestion tank 21, the digestion gas produced during the digesting step is recovered by a gas holder(not shown) through a degassing pipe 22. And then, the digesting gas is boosted in pressure by a blower (not shown) and it is injected under a predetermined pressure to the vicinity of the tank bottom via an agitating pipe 23. As a result, the sludge in the tank is agitated by digestion gas so that the organisms in the sludge may be in organized and or gas if led by the action of an anaerobe such as facultative anaerobe or methane fermentum.
The digested sludge which has been reduced, stabilized and made safe by the inorganizing and the gasifying treatments, is discharged through a digested sludge discharge pipe 24 which is disposed at the bottom center of the tank.
However, the prior art construction of the agitating apparatus, which includes the draft tube 2 erected in the digestion tank 1 and the agitator supported by a concrete slab 8 at the top of the digestion tank 1 and in which the screw 4 connected to the motor 11 of the agitator 3 is inserted into the upper open end of the draft tube 2, is a widely adopted technology. This construction presupposes that the digestion tank has been designed in advance to have a strength capable of disposing the agitator 3 in the concrete slab 8 of the top of the digestion tank 1. However, the construction has a disadvantage that it cannot be installed from the standpoint of strength on the digestion tank which has failed to take such consideration in the designed strength or on the existing digestion tank(of the type, as shown in FIG. 7).
On the other hand, the latter construction (of FIG. 7), in which the sludge 5 in the tank is agitated by injecting the digestion gas produced at the digesting step to the vicinity of the tank bottom, is inferior in agitating effect of FIG. 5, i.e., the agitating efficiency of the mechanical agitation type.
Thus, there is a tendency of increasing the need for replacing the agitating construction of the latter digestion tank 21, see FIG. 7, in which the agitation is performed by the already running gas or pump, with a mechanical agitation type. However, the following problems arise in case the agitator of FIG. 5 is installed in an existing agitation tank 21:
1 The top slab of the existing digestion tank 21 may frequently have such a small thickness as to lack the strength for installing the agitator 3 or fail to maintain the strength of the concrete after one decade or two decades even if the strength was sufficient when designed. Thus, the agitator 3 cannot be safely installed on the top slab of the existing digestion tank 21.
2 In this case, however, the installation could be made if the agitator 3 were placed on a strong support disposed inside or outside of the existing digestion tank 21, to bear the load and if the gas sealing of the top slab, through which the agitator 3 passes, were sufficiently devised. However, the construction is so large-sized and complicated as to raise the cost for the improvement.
3 These problems could be solved by disposing the agitator in the digestion tank. Since, however, the digestion tank is a unit for producing combustible gases, the inner construction is not easy to open and inspect. It is a common practice to perform the required maintenance work about once every five years, even if other maintenance work is needed in the interim. Thus, good maintenance cannot be sufficiently achieved, and a long down time and high cost are required, if problems should occur, which require inspections and repairs.